The digital electronic communication medium is becoming an increasingly common substitute for traditional forms of communication, such as the hard-copy mail medium. For example, the Internet is a common medium for electronic mail "E-mail" messages between humans. In fact, the humorously disparaging term "snail mail" has recently been used to refer to traditional hand-delivered hard-copy mail services.
However, hard-copy communication media, such as hard-copy mail service provided by the United States Postal Service, are alive and well. Moreover, the development of electronic communication has, in some ways, been beneficial for traditional hard-copy mailing. Certain differences between electronic and hard-copy forms of communication are becoming more apparent, and it is possible to recognize advantages and disadvantages of one over the other. At the same time, by recognizing an advantage of one medium, it is possible to develop previously unused ways of realizing similar advantages in the other
Consider, for instance, the general field of authentication of posting of a message. Hard-copy mail has traditionally provided, through postmarking of a piece of mail, a representation that the mail was posted on the date of postmarking. Additionally, mail may be certified, at the request of the sender and for a suitable fee. Thus, certain types of important mail which must be mailed by a required date, such as Internal Revenue Service tax forms, can be certified as to the date of posting.
However, this familiar process of certifying a piece of hard-copy mail has a significant limitation. Certification as described above proves that some item of mail was posted on the day of posting. It does not prove, however, that any particular item of mail, having any particular content, was posted. To accomplish this latter proof, it is necessary to certify the content of the document.
Certain types of certification have been made available in the context of electronic communication. For example, the related problem of certifying the identity of a sender has been dealt with. See, for instance, Fischer, U.S. Pat. No. 4,868,877, "Public Key/Signature Cryptosystem with Enhances Digital Signature Certification". However, the Fischer '877 patent pertains only to electronic communication and cryptography, and does not address the problem of certifying either content in general, or content of a hard-copy document in particular.
Thus, there is a drawback to the certification of hard-copy mail, in that the content of the mail item cannot be certified. This drawback has not been adequately solved by existing techniques, such as those discussed above.